Communication systems use antennas for transmitting and receiving communication signals. The communication systems can use a variety of antenna systems having transmitter and receiver antennas for defining antenna gain patterns with peaks for directional transmitting and receiving the communication signals. One type of antenna system is the active transmit phased arrays having multiple directional antenna elements using beam steering. Typically, the phased array antenna has a plurality of individual antenna elements lying in plane. Each antenna element broadcasts one or more steered communication signals eliminating the need for multiple apertures. Each array element has a respective phase offset for each signal for steering the respective antenna beams in a desired direction toward communication receivers.
Active transmit phased array antenna typically have one main beam using a beam forming design in which all of the elements of phased array antenna are phased to collectively point the main beam in a desired direction. Active phased arrays have solid-state power amplifiers at each array element. These solid-state power amplifiers are nonlinear devices that produce the unwanted intermodulation products when multiple signals are introduced and communicated. The intermodulation frequencies are spaced according to the difference between the frequencies. For example, when two transmit carrier frequencies f1 and f2 are used for broadcasting signals with two primary main beams, then the unwanted intermodulation product frequencies are 2f1–f2 and 2f2–f1. The phased array produces antenna patterns at the intermodulation frequencies. The secondary intermodulation main beams of the intermodulation product patterns are steered according to the difference in the pointing angles of the primary main beams. Therefore, the phased array antenna field of view contains the two primary main beams and may contain intermodulation grating lobe beams depending on the difference in pointing angles of the two primary main beam patterns. When the primary main beams are closely spaced, then the secondary intermodulation main beams and intermodulation grating lobe beam may disadvantageously appear within the field of view of the phased array antenna. When the primary main beams are widely spaced, then a special condition occurs where the secondary intermodulation main beams advantageously appear outside the field of view and the intermodulation grating lobe beams disadvantageously appear within the field of view. When the intermodulation grating lobe beams are in the field of view, then the intermodulation grating lobe beams are unwanted interference generated at the intermodulation frequencies. In theory, the beam forming design can be modified to accommodate two or more main beams. However, the transmission of the multiple communication signals create unwanted intermodulation products in power amplifiers that produce gain patterns appearing as unwanted signals at intermodulation frequencies in secondary intermodulation main beams and grating lobes in the antenna gain pattern. As a practical consideration, extensive design modifications are necessary to produce two or more beams with an active transmit phased array antenna. For example, phased array antennas having a plurality of main beams and operating at a plurality of different frequencies for transmitting a plurality of different signals produce intermodulation products as unwanted intermodulation beams typically because solid-state power amplifiers at each array element produce intermodulation products when multiple signals are transmitted or received.
Primary techniques for reducing intermodulation products include backing off the output amplifier from a highest level, and linearization of the output high power amplifier. For example, a 1 dB power backoff of the amplifier would yield a 3 dB suppression of the intermodulation product. Power backoff reduces the amplifier power efficiency and has limited effectiveness, and amplifier linearization requires costly complexity in the antenna design. Transmitter power amplifier linearizers and power back-off methods are used to reduce signal distortion. While solid-state power amplifier linearizers and power back-off techniques can lower the levels of the unwanted intermodulation products, such techniques lower the array efficiency. The phased arrays have intermodulation products that produce unwanted beams because solid-state power amplifiers at each array element produce intermodulation products when multiple signals are introduced. Primary methods for reducing intermodulation products are amplifier output-power-backoff and amplifier linearization. Power backoff reduces the amplifier power efficiency and has limited effectiveness, and amplifier linearization requires costly development work.
Hence, it is desirable to control the phased array elements with grating lobe suppression for reduced signal distortion during signal transmission that may use saturated power amplifiers and linearization methods. It is desirable to use a phasing method that reduces unwanted intermodulation products in addition to linearization methods. However, an improved phasing method should be independent of nonlinear high power amplifiers or solid-state power amplifiers. In a subarray separation approach, the phased array antenna elements are first divided into equal subarrays and then each subarray is separated by uniform spacing from each other so as to reduce intermodulation products. By physically partitioning the array into subarrays of antenna array elements and then physically separating the subarrays, unwanted grating lobes in the field of view were suppressed. The subarray separation approach disadvantageously only suppresses intermodulation grating lobe beams and does not suppress all undesirable intermodulation product beams, for example, the intermodulation main beams. These and other disadvantages are solved or reduced using the invention.